
# == title
# Draw arrow which is paralle to the circle
#
# == param
# -x1 start position of the arrow on the x-axis.
# -x2 end position of the arrow on the x-axis.
# -y position of the arrow on the y-axis. Note this is the center of the arrow on y-axis.
# -width width of the arrow body.
# -sector.index index of the sector.
# -track.index index of the track.
# -arrow.head.length length of the arrow head. Note the value should be smaller than the length of the arrow itself (which is ``x2 - x1``).
# -arrow.head.width width of the arrow head.
# -arrow.position where is the arrow head on the arrow.
# -tail the shape of the arrow tail (the opposite side of arrow head).
# -border border color of the arrow.
# -col filled color of the arrow.
# -lty line style of the arrow.
# -... pass to `graphics::polygon`.
#
# == details
# Note all position values are measured in the data coordinate (the coordinate in each cell).
# 
# If you see points overflow warnings, you can set ``circos.par(points.overflow.warning = FALSE)`` to turn it off.
#
# == author
# Zuguang Gu <z.gu@dkfz.de>
#
# == example
# circos.initialize(letters[1:4], xlim = c(0, 1))
# circos.track(ylim = c(0, 1), panel.fun = function(x, y) {
# 	circos.arrow(0, 1, y = 0.5, width = 0.4, arrow.head.length = ux(1, "cm"), 
# 		col = "red", tail = ifelse(CELL_META$sector.index \%in\% c("a", "c"), 
# 			"point", "normal"))
# }, bg.border = NA, track.height = 0.4)
#
# ########## cell cycle ###########
# cell_cycle = data.frame(phase = factor(c("G1", "S", "G2", "M"), 
#                                     levels = c("G1", "S", "G2", "M")),
# 	                      hour = c(11, 8, 4, 1))
# color = c("#66C2A5", "#FC8D62", "#8DA0CB", "#E78AC3")
# circos.par(start.degree = 90)
# circos.initialize(cell_cycle$phase, xlim = cbind(rep(0, 4), cell_cycle$hour))
# circos.track(ylim = c(0, 1), panel.fun = function(x, y) {
# 	circos.arrow(CELL_META$xlim[1], CELL_META$xlim[2], 
# 		arrow.head.width = CELL_META$yrange*0.8, arrow.head.length = ux(1, "cm"),
# 		col = color[CELL_META$sector.numeric.index])
# 	circos.text(CELL_META$xcenter, CELL_META$ycenter, CELL_META$sector.index, 
# 		facing = "downward")
# }, bg.border = NA, track.height = 0.3)
# circos.clear()
#
circos.arrow = function(x1, x2, y = get.cell.meta.data("ycenter", sector.index, track.index), 
	width = get.cell.meta.data("yrange", sector.index, track.index)/2, 
	sector.index = get.current.sector.index(), track.index = get.current.track.index(),
	arrow.head.length = convert_x(5, "mm", sector.index, track.index),
	arrow.head.width = width*2, arrow.position = c("end", "start"),
	tail = c("normal", "point"), border = "black", col = "white", lty = par("lty"), ...) {

	arrow.position = match.arg(arrow.position)[1]
	tail = match.arg(tail)[1]

	sector.index = sector.index
	track.index = track.index

	if(x2 <= x1) {
		stop("`x2` should be larger than `x1`. Set `arrow.position = 'start'`\nto get reverse clockwise arrows.")
	}
	
	if(abs(x2 - x1 - arrow.head.length) < 1e-6) {
		stop("Arrow head is too long that it is even longer than the arrow itself.")
	}

	if(arrow.position == "end") {
		arrow.head.coor = rbind(c(x2 - arrow.head.length, y + arrow.head.width/2),
			                    c(x2, y),
			                    c(x2 - arrow.head.length, y - arrow.head.width/2))
		if(tail == "normal") {
			arrow.body.coor = rbind(c(x2 - arrow.head.length, y - width/2),
				                    c(x1, y - width/2),
				                    c(x1, y + width/2),
				                    c(x2 - arrow.head.length, y + width/2))
			arrow.body.coor2 = rbind(lines.expand(arrow.body.coor[1:2, 1], arrow.body.coor[1:2, 2], sector.index, track.index),
				                    lines.expand(arrow.body.coor[3:4, 1], arrow.body.coor[3:4, 2], sector.index, track.index))
		} else {
			arrow.body.coor = rbind(c(x2 - arrow.head.length, y - width/2),
				                    c(x1, y),
				                    c(x2 - arrow.head.length, y + width/2))
			arrow.body.coor2 = rbind(lines.expand(arrow.body.coor[1:2, 1], arrow.body.coor[1:2, 2], sector.index, track.index),
				                    lines.expand(arrow.body.coor[2:3, 1], arrow.body.coor[2:3, 2], sector.index, track.index))
		}

		coor = rbind(arrow.body.coor2, arrow.head.coor)
	} else {
		
		arrow.head.coor = rbind(c(x1 + arrow.head.length, y + arrow.head.width/2),
			                    c(x1, y),
			                    c(x1 + arrow.head.length, y - arrow.head.width/2))
		if(tail == "normal") {
			arrow.body.coor = rbind(c(x1 + arrow.head.length, y - width/2),
				                    c(x2, y - width/2),
				                    c(x2, y + width/2),
				                    c(x1 + arrow.head.length, y + width/2))
			arrow.body.coor2 = rbind(lines.expand(arrow.body.coor[1:2, 1], arrow.body.coor[1:2, 2], sector.index, track.index),
				                    lines.expand(arrow.body.coor[3:4, 1], arrow.body.coor[3:4, 2], sector.index, track.index))
		} else {
			arrow.body.coor = rbind(c(x1 + arrow.head.length, y - width/2),
				                    c(x2, y),
				                    c(x1 + arrow.head.length, y + width/2))
			arrow.body.coor2 = rbind(lines.expand(arrow.body.coor[1:2, 1], arrow.body.coor[1:2, 2], sector.index, track.index),
				                    lines.expand(arrow.body.coor[2:3, 1], arrow.body.coor[2:3, 2], sector.index, track.index))
		}
		coor = rbind(arrow.body.coor2, arrow.head.coor)
	}
	coor = rbind(coor, coor[1, ])
	
	d2 = circlize(coor[, 1], coor[, 2], sector.index, track.index)
	polygon(polar2Cartesian(d2), border = border, col = col, lty = lty, ...)
}
